1. Field of the Invention
The present invention relates to an electric double layer capacitor using an active carbon.
2. Related Art
An electric double layer capacitor has a wide usable temperature range and high power density. In order to improve energy density, it is necessary to increase the use voltage of a capacitor with an active carbon. A method in which active carbon having high capacitance for electric double layer capacitors is produced by alkali-activating easily graphitized carbon material is known (Japanese Unexamined Patent Application Publication No. H9-275042). As exemplary methods for producing active carbon, alkali activation methods for carbon materials are known (Japanese Unexamined Patent Application Publications Nos. 2002-15958, 2002-134369, H1-139865, and H10-121336).
However, the alkali activated active carbons have problems in that durability in nonaqueous organic electrolyte solutions such as propylene carbonate (PC) solution is remarkably deteriorated, and in that large quantities of gas are generated by dissociating solvent. In particular, when voltage is raised by using an electrolyte solution which consists mainly of PC solution, the electrolyte solution is dissociated so as to generate gas and internal pressure in an aluminum case of the capacitor is increased, and many inconveniences thereby occur.
In addition, it is known that in durability tests for temperature and voltage load, characteristics are greatly deteriorated and resistance increase rate is increased when pore volume of the active carbon is 0.3 ml/g or less, and pore structures, surface states of carbon, ranges of low metal impurity amounts, and exemplary production methods thereof, which are suitable for improving the durability, have been reported (Japanese Unexamined Patent Application Publications Nos. 2002-83747, 2003-86469, 2004-47613, and 2004-67498).
Furthermore, it is known that in the electric double layer capacitor, stable high voltage resistance is obtained by using an anode and a cathode in which polarizing electrode amounts of each electrode differ (Japanese Unexamined Patent Application Publication No. S61-203614). However, when the electrode amount of the cathode is decreased, the capacitor is not useful since capacitance is reduced. In addition, as an active carbon for a polarizing electrode used in an anode and a cathode, carbon materials in which pore diameters differ are used in an anode and a cathode in order to increase capacitance have been proposed (Japanese Unexamined Patent Application Publication No. H11-67608); however, static capacitance is small and sufficient characteristics are not exhibited since the average of each pore diameter of the active carbon is 1.1 nm and 1 nm and the specific surface area is 1500 m2/g or more.
Therefore, in the electric double layer capacitor using conventional active carbon as described above, characteristics of static capacitance, internal resistance and dissociating of electrolyte solution are not sufficient, and further improvement of durability is required.